Cylindrical stents to be inserted to be placed for use in in vivo organs having tubular structures (e.g., blood vessels, esophagus, and trachea) are used in order to expand or maintain tubular structures for treatment of aneurysm, thrombosis, or the like.
Most in vivo tubular structures are complex such as in the cases of bending structures. Hence, the insertion of a stent into a site having such a complicated structure requires mechanical flexibility.
For example, a brain aneurysm is a type of aneurysmal vascular lesion that is generated on the arterial wall of a cerebral blood vessel. A brain aneurysm rupture causes the onset of cerebral hemorrhages such as subarachnoid hemorrhages that result in high mortality rates. Stents are also used in intravascular operations for such brain aneurysms. Among in vivo organs having tubular structures, the cerebrovascular system has a particularly complicated structure, in which many sites characterized by a large degree of curvature are present. When a stent is inserted into such a site, the stent is required to have a particularly high degree of mechanical flexibility.
A stent is generally formed of biocompatible metal wires that form a specific mesh pattern (network structure). There are mesh patterns having open cell structures, wherein mesh-composing cells (openings) and adjacent cells do not share some sides or vertices and mesh patterns having closed cell structures, wherein mesh-composing cells (openings) and adjacent cells share all sides and vertices. Stents having open cell structures have a high degree of flexibility because of their structures and thus are useful. An example thereof is a Neuroform Stent (Neuroform Micro delivery Stent System, Boston Scientific/, Fremont, Calif.) (see Sepehr Sani. Et al., Nurosurg Focus 18 (2): E4, 2005, p. 1-5). However, concerns have been raised that in a stent having an open cell structure, some wires (struts) may protrude outside the stent when it is bent, so as to injure in vivo tubular tissue such as the tissue of a blood vessel into which the stent has been inserted to be placed.
On the other hand, concerns that some wires (struts) may protrude outside the stent have not been raised for a stent having a closed cell structure (see JP Patent Publication (Kokai) No. 2003-93518 A, JP Patent Publication (Kokai) No. 2003-93519 A, and Randall T. Higashida et al., AJNR Am J Neuroradiol 26: 1751-1756, August 2005, p. 1751-1756). However, stents having closed cell structures may have poor flexibility because of their structures. Hence, there has been doubt about the usefulness of such a stent for the cerebrovascular system, which requires a particularly flexible stent. However, the relationship between a mesh pattern having a closed cell structure and flexibility has not yet been analyzed.